This invention relates to novel methods for the sulfonation of anthraquinones, and particularly to the sulfonation of anthraquinone in a sulfur dioxide solvent.
Anthraquinone derivatives are among the leading dyestuffs used commercially. Many of these derivatives are produced by processes beginning with the sulfonation of anthraquinone to form monosulfonic acids, disulfonic acids, mixtures thereof and salts thereof. Such monosulfonic acids include principally 1-anthraquinonesulfonic acid (alpha), 2-anthraquinonesulfonic acid (beta). Disulfonic acids include anthraquinone-1,5-disulfonic acid, anthraquinone-1,8-disulfonic acid (alpha), anthraquinone-2,6-disulfonic acid and anthraquinone-2,7-disulfonic acid (beta). Anthraquinone-1,7-disulfonate, and other possible anthraquinone sulfonates or disulfonates are not usually formed in significant quantities.
Sulfonation of anthraquinone is normally conducted by adding anthraquinone to oleum (fuming sulfuric acid) as solvent and reagent and a catalyst such as mercury (usually as the sulfate) and/or boric acid. Sulfonation without a mercury catalyst gives beta acids almost exclusively (2-sulfonic, 2,6-disulfonic and 2,7-disulfonic) while addition of small amounts of mercury is known to give alpha subsitution (1-sulfonic, 1,5-disulfonic and 1,8-disulfonic).
After sulfonation with oleum, the sulfonic acid is conventionally quenched by drowning the batch in water and precipitating the acid as an alkali metal salt. Examples of such processes are disclosed in: U.S. Pat. Nos. 2,742,484, 2,900,397, 2,999,869, and 3,079,404; British Pat. Nos. 1,314,120 and 1,339,114; French Pat. No. 2,139,530; German Pat. No. 2,163,674; Organic Chemistry of Sulfur by C. M. Suter, pages 302 to 305 and Kirk-Othmer, Encyclopedia of Chemical Technology, pages 439 to 447. United States Patent Nos. 2,074,307 and 2,074,309 report a lesser dilution of the batch with water and precipitation of the acid as the oxonium salt.
Sulfonation with oleum as both the sulfonating agent and the solvent presents serious pollution and economic disadvantages. Substantial amounts of sulfuric acid are discarded in dilute form after precipitation of the sulfonates. Particularly after the preparation of the alpha sulfonates 1-, 1,5- and 1,8-, where mercury catalyst is used, the sulfuric acid by-product is contaminated with mercury which makes it both unusable in other processes and extremely polluting if discharged into waterways. Attempts to remove the mercury impurities are both expensive and, in some cases, incomplete.